In the road construction and road paving industry, it is a well-practised procedure to coat aggregate material such as sand, gravel, crushed stone or mixtures thereof with hot fluid bitumen, spread the coated material as a uniform layer on a road bed or previously built road while it is still hot, and compact the uniform layer by rolling with heavy rollers to form a smooth surfaced road.
The combination of bitumen with aggregate material, such as sand, gravel, crushed stone or mixtures thereof, is also referred to as “asphalt”. Bitumen, also referred to as “asphalt binder”, is usually a liquid binder comprising asphaltenes, resins and solvents. Bitumen can for example comprise pyrogenous mixtures derived from petroleum residues such as residual oils, tar or pitch or mixtures thereof.
It is known in the art that sulphur can be mixed with bitumen for applications in the road construction and road paving industry.
In US 2003/073761 A1 a composition comprising sulphur, a thermoplastic elastomer and tetramethylhiuram disulfide is described.
In GB-A-2,384,240 a composition comprising sulphur in the range of from 0.01 to 2 wt %) is described.
In DE 110,58,712 A1 a composition comprising 18.4 wt % of elemental sulphur) is described.
Efforts towards improving the addition of sulphur to bitumen are for example described in GB 1,528,384. More recently, studies on the use of sulphur in bituminous mixtures have indicated that the use of sulphur-modified bitumen is feasible. Sulphur-modified bitumen is formulated by replacing some of the bitumen in conventional binders by elemental sulphur.
One of the problems encountered when using sulphur in bitumen is the unwanted formation of H2S, resulting from dehydrogenation reactions between bitumen and sulphur at high temperatures.
Even low H2S emission from sulphur-comprising asphalt, meaning asphalt formulated using sulphur-modified bitumen wherein elemental sulphur has been used to replace part of the bitumen, presents an emission nuisance on road paving projects. This is due to the gradual H2S gas concentration increase to high levels in the air voids in the loose paving mixture during storage in silos and during truck delivery to the paving site. The “stored” gas is released when the air pockets in the mixture are opened up as the mixture is dumped from the delivery trucks or as the mixture is subjected to mechanical mixing.
In view of the substantial amounts of sulphur used, especially in sulphur-comprising asphalt having high sulphur-bitumen weight ratios, e.g. as high as 1:1, H2S emission is a serious problem. Therefore, it is necessary to reduce the unwanted formation and emission of H2S from sulphur-comprising asphalt.
In Patent abstracts of Japan, vol. 2000, no. 20, 2001-07-10 a method for reducing H2S formation during vulcanization is described. No reference is made to sulphur-bitumen mixtures.
One method to reduce H2S-emission from hot cast sulphur-asphalt mixtures is to add an H2S-suppressant in the process to manufacture sulphur-bitumen mixtures by mixing and heating sulphur and bitumen in the presence of added H2S-suppressant as described in U.S. Pat. No. 3,960,585 and in EP-A-121,377.
A disadvantage of the method described in U.S. Pat. No. 3,960,585 is that liquid H2S-suppressant has to be injected into paving mixture at the mixing plant. Consequently, equipment for injecting has to be set up and maintained at the mixing plant, making the process cumbersome and costly. Another disadvantage is that it is more difficult to achieve a homogeneous distribution of H2S-suppressant in the paving mixture, as a relatively small amount of liquid H2S-suppressant is added to a relatively large mixture of solids and liquids.